<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        .box {
            height: 200px;
            display: flex;
            background: green;
            flex-flow: row wrap;
            justify-content: space-around; /** 间隔 平分空闲区域**/
            /**	 align-items: flex-start | flex-end | center | baseline | stretch; **/
            /**  align-items: flex-start; 容器顶部对齐**/
            /**  align-items: flex-end; 容器底部对齐**/
            /** align-items:center;   容器对齐 Y轴中线对齐**/
            /**     align-items:baseline; 项目的第一行文字的基线对齐。**/
          /**  stretch（默认值）：如果项目未设置高度或设为auto，将占满整个容器的高度。**/
        }

        body {
            margin: 0;
            padding: 0;
        }

        .item1 {
            background: red;
            height: 500px;

        }

        .item2 {
            background: yellow;
        }

        .item3 {
            background: blue;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="item1">1</div>
    <div class="item2">2</div>
    <div class="item3">3</div>
    <div class="item1">4</div>


</div>
</body>
</html>